In a conventional plasma ignition technique, spark discharge is generated between the electrodes of a spark plug by means of DC power, and in this state AC plasma is generated between the electrodes by means of AC power (see, for example, Japanese Patent Application Laid-Open (kokai) No. S51-77719 “Patent Document 1” and Japanese Patent Application Laid-Open (kokai) No. 2009-36198 “Patent Document 2”). Also, there has been proposed a technique of increasing stepwise AC power during generation of AC plasma in order to expand the AC plasma (see, for example, Pamphlet of WO2009/147335 “Patent Document 3”).